The present invention relates to hydraulic antivibration sleeves.
More particularly, the invention relates to a hydraulic antivibration sleeve comprising:
a rigid inner strength member extending longitudinally along a central axis between two axial ends;
an annular rigid outer strength member which surrounds the inner strength member;
an elastomer body interconnecting the inner and outer strength members, said elastomer body comprising:
a first thick elastomer wall adapted to support a permanent load of value lying in a certain operating domain and applied between the inner and outer strength members parallel to a main vibration direction perpendicular to said central axis, said first thick elastomer wall presenting a central portion secured to the inner strength member and a peripheral portion secured to the outer strength member, thereby defining part of a first liquid-filled working chamber; and
a second thick elastomer wall having a peripheral portion secured to the outer strength member and defining a second liquid-filled working chamber;
a liquid-filled compensation chamber defined by at least a first flexible elastomer wall that is easily deformable and thinner than said first and second thick elastomer walls;
a first constricted passage putting said first working chamber into communication with the compensation chamber; and
a second constricted passage putting the second working chamber into communication with one of the other two chambers, the first and second constricted passages being dimensioned to present respective first and second different resonant frequencies.
Document EP-A-0 411 997 describes an element of such a sleeve in which the second working chamber is defined in part by the first thick wall of the elastomer body, while the second thick wall of the elastomer body separates the two working chambers. In that known antivibration sleeve, the compensation chamber which communicates with both working chambers is placed diametrically opposite the working chamber, and the flexible elastomer walls which define said compensation chamber is integrally formed with the elastomer body. The flexible wall is independent of the inner strength member and presents a peripheral portion secured to the outer strength member.
That prior art antivibration sleeve gives entire satisfaction in that the two working chambers and the two constricted passages provide particularly effective damping of vibration in the vicinity of the two resonant frequencies of the constricted passages.
Nevertheless, the operation of such known antivibration sleeves is independent of the static load applied between the inner and outer strength members. Unfortunately, in some cases, it can be useful for the antivibration sleeve to operate differently depending on the static load applied between the two strength members.
A particular object of the invention is to resolve this technical problem.
To this end, according to the invention, an antivibration sleeve of the kind in question is characterized in that the second working chamber is diametrically opposite to the first working chamber about the central axis, the two working chambers being substantially in alignment along the main direction of vibration, the second thick elastomer wall presenting a free central portion which is disposed in the vicinity of the inner strength member and which is independent of said inner strength member; and
in that the elastomer body is so dimensioned:
that the free central portion of the second elastomer wall is in contact with the inner strength member when said static load lies in a first range of values belonging to said operating domain; and
that the free central portion of the second elastomer wall is separate from the inner strength member by an empty space when the static load lies in a second range of values complementary to the first range of values in said operating domain.
By means of these dispositions, when the static load lies in the first range of values, relative vibratory motion between the two strength members of the sleeve gives rise to deformation in the two working chambers and thus to transfers of liquid between each of the working chambers and the corresponding chamber via the constricted passages: in this first mode of operation, particularly effective damping is obtained, in particular for frequencies of vibration close to the second resonant frequency.
In contrast, when the static load lies in the second range of values, only the first working chamber is influenced by the relative vibratory motion between the two strength members of the sleeve, such that the transfers of liquid take place mainly between said first working chamber and the compensation chamber via the first constricted passage: in this second mode of operation, vibrations are damped effectively at frequencies that are close to the first resonant frequency.
In preferred embodiments of the invention, use can optionally be made of one or more of the following dispositions:
the second constricted passage interconnects the first and second working chambers;
the compensation chamber is adjacent at least to the second working chamber and is partially defined by the second thick elastomer wall;
the compensation chamber is generally U-shaped and comprises:
first and second side portions which are disposed on either side of the second working chamber and which are defined in part at least by the second thick elastomer wall, said first and second side portions being defined towards the axial ends of the inner strength member respectively by a first flexible elastomer wall and by a second flexible elastomer wall, each of the first and second flexible elastomer walls being connected at least to the outer strength member; and
a central portion which extends substantially parallel to the central axis between said two side portions;
the first and second flexible elastomer walls are molded integrally with the second thick elastomer wall;
the empty space between the second thick elastomer wall and the inner strength member is filled with air and communicates with the atmosphere;
the first and second flexible elastomer walls are annular pieces distinct from the elastomer body, each of which pieces is connected to the inner and outer strength members, the side portions of the compensation chamber being annular and adjacent to the two working chambers, and the central portion of said compensation chamber comprising the empty space between the second thick elastomer wall and the inner strength member;
the elastomer body has a solid central portion which extends angularly around the inner strength member from the first working chamber to the second working chamber, the second constricted passage being formed in said solid central portion; and
the central portion of the compensation chamber is angularly connected to the first working chamber via an additional passage formed between the elastomer body and the outer strength member, said passage being normally closed by a flexible elastomer rib projecting radially outwards from the elastomer body to a free edge placed in contact with the outer strength member, said flexible rib being dimensioned to fold down and open said additional passage between the central portion of the compensation chamber and the first working chamber when the pressure difference between these chambers is greater than a predetermined value.